Electrically conductive walled hollow metal waveguides, both rectangular and elliptical, have long served as a preferred form of transmission line through which to propagate high frequency electromagnetic energy RF, microwaves, between spaced locations. The electronic characteristics of those lines are well defined mathematically, the results predictable and the techniques to their construction well known. For most transmission applications such microwave waveguides are of a rigid structure. However, in microwave systems in which mechanical rotation is required of one part of the system, relative to another part held fixed in position, the transition between the two is often accomplished with a flexible waveguide: As example, one part of such a microwave system may be a microwave radar transmitter or transceiver that is maintained stationary; the second or movable part may be the microwave antenna, which is reciprocated over a predetermined arc to electronically "look" in different directions. Typically, the antenna is reciprocated back and forth over a wide arc continuously to electronically maintain surveillance over a prescribed region.
A length of waveguide typically includes a coupling device secured to each end, referred to as an end flange, or, simply, a flange. Those flanges contain bolt holes and a microwave window permitting the flanges to be bolted to like flanges of another device or waveguide, completing the microwave passage into the latter devices and thereby linking the waveguide into a microwave system, such as the microwave radar transmitter and the antenna previously referred to. Although the waveguide may be flexible in structure, the end flanges are necessarily quite sturdy and rigid, as is the attachment of those flanges with the waveguide's flexible metal walls.
The techniques of manufacturing flexible microwave waveguides so as to impart a characteristic flexibility that permits it to be bent or curved around a corner or the like, and that of the end flange construction and the flange's attachment to the waveguide are well known and require no description. Although the present invention, makes use of flexible waveguide and flanges, the construction details thereof are not material to an understanding of the invention and need not be described in detail.
Simple arcuate movement of one end of a length of flexible waveguide over a fixed axis of rotation while holding the other end stationary causes the effective length of the waveguide to change. The length is shortened, causing the waveguide to become compressed. That change is physically possible due to the flexible structure of the waveguide's construction. However, the large change in length, about ten per cent, occurring at large angles of rotation, such as sixty degrees or beyond from a straight configuration, induces high strains and stresses in the waveguide. That increment of length must be absorbed within the structure of the flexible construction or results in distortion of the shape of the waveguide as may cause buckling and large strains on the end flange connections with the waveguide.
The strain of repeated bending and unbending of the waveguide in normal operation ultimately causes the waveguide to mechanically break, setting a limit to the operation of the system, as may be expressed in terms of a number of bending cycles, at which time waveguide replacement is required. With conventional flexible waveguide construction, that high strain often causes the waveguide to buckle and fail prematurely, in less than 20,000 bending cycles. That is a factor of fifty times less than would occur when repeatedly bending only over small angles, where the change in waveguide length is very small and the accompanying mechanical stress insignificant.
In a prior invention, I've demonstrated that it is possible to reduce such high strains and stresses by controlling the shape of the bend during the arcuate movement of the waveguide end so as to minimize or avoid changing the waveguide's length. In my prior patent U.S. Pat. No. 5,289,710 granted Mar. 1, 1994, entitled Apparatus for Bending a Flexible Conduit, I describe a bending device and method for achieving such rotation that does not generate mechanical forces that augur change in the waveguide's length. That prior bending apparatus requires several special components to achieve the benefit of greater reliability. As inspection of disclosure of that patent reveals a bending apparatus that employs two variable length arms and a single rotation or pivot axis. An optimum rotation axis-to-flange distance, ie. the effective length of the arms, is the only variable parameter. The angle of rotation around the pivot point is the same as the angle of rotation of the moving flange.
As one appreciates, the structure of my prior bending device requires specially manufactured components, not available, so to speak, off-the-shelf. Further, for greatest operational life with that structure, it is found that the maximum range of angular rotation of the moveable flange should be no greater than ninety degrees. For a greater angular excursion larger sized components should be used, which poses other disadvantage and is not desirable.
Although the foregoing apparatus offers considerable benefit over the bending devices that preceded, the need for improvement, particularly in enhancing the permissible range of its angular excursion without concomitant reduction in the apparatus's operational life, is evident. The incentive thus exists to devise a less complicated and more easily assembled conduit bending apparatus which, at a minimum, equals the benefits provided by my earlier bending mechanism. As an advantage, the present invention provides a simpler solution and, yet, surprisingly, offers even greater advantage than my prior invention.